The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for constant current control with primary-side sensing and regulation in various operation modes. Merely by way of example, the invention has been applied to a flyback power converter. But it would be recognized that the invention has a much broader range of applicability.
Generally, a conventional power conversion system often uses a transformer to isolate the input voltage on the primary side and the output voltage on the secondary side. To regulate the output voltage, certain components, such as TL431 and an opto-coupler, can be used to transmit a feedback signal from the secondary side to a controller chip on the primary side. Alternatively, the output voltage on the secondary side can be imaged to the primary side, so the output voltage is controlled by directly adjusting some parameters on the primary side.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system with primary-side sensing and regulation. The power conversion system 100 includes a primary winding 110, a secondary winding 112, an auxiliary winding 114, a power switch 120, a current sensing resistor 130, an equivalent resistor 140 for an output cable, resistors 150 and 152, and a rectifying diode 160. For example, the power switch 120 is a bipolar transistor. In another example, the power switch 120 is a MOS transistor.
To regulate the output voltage within a predetermined range, information related to the output voltage and the output loading often needs to be extracted. In the power conversion system 100, such information can be extracted through the auxiliary winding 114. When the power switch 120 is turned on, the energy is stored in the secondary winding 112. Then, when the power switch 120 is turned off, the stored energy is released to the output terminal, and the voltage of the auxiliary winding 114 maps the output voltage on the secondary side as shown below.
                              V          FB                =                                                            R                2                                                              R                  1                                +                                  R                  2                                                      ×                          V              aux                                =                      k            ×            n            ×                          (                                                V                  o                                +                                  V                  F                                +                                                      I                    o                                    ×                                      R                    eq                                                              )                                                          (                  Equation          ⁢                                          ⁢          1                )            
where VFB represents a voltage at a node 154, and Vaux represents the voltage of the auxiliary winding 114. R1 and R2 represent the resistance values of the resistors 150 and 152 respectively. Additionally, n represents a turns ratio between the auxiliary winding 114 and the secondary winding 112. Specifically, n is equal to the number of turns of the auxiliary winding 114 divided by the number of turns of the secondary winding 112. Vo and Io represent the output voltage and the output current respectively. Moreover, VF represents the forward voltage of the rectifying diode 160, and Req represents the resistance value of the equivalent resistor 140. Also, k represents a feedback coefficient as shown below:
                    k        =                              R            2                                              R              1                        +                          R              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
FIG. 2 is a simplified diagram showing a conventional operation mechanism for the flyback power conversion system 100. As shown in FIG. 2, the controller chip of the conversion system 100 uses a sample-and-hold mechanism. When the demagnetization process on the secondary side is almost completed and the current Isec of the secondary winding 112 almost becomes zero, the voltage Vaux of the auxiliary winding 114 is sampled at, for example, point A of FIG. 2. The sampled voltage value is usually held until the next voltage sampling is performed. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref. Therefore,VFB=Vref  (Equation 3)
Combining Equations 1 and 3, the following can be obtained:
                              V          o                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              o                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          4                )            
Based on Equation 4, the output voltage decreases with the increasing output current.
Additionally, in the discontinuous conduction mode (DCM), the flyback power conversion system 100 can also regulate the output current regardless of the output voltage based on information associated with the waveform for the voltage Vaux of the auxiliary winding 114 as shown in FIG. 2.
FIG. 3 is a simplified conventional diagram showing characteristics of output voltage and output current of a flyback power conversion system. As shown in FIG. 3, if the output current Io is in the range of from zero to Imax, the system operates in the constant voltage (CV) mode. In the CV mode, the output voltage Vo is equal to Vmax. Alternatively, if the output voltage is below Vmax, the system operates in the constant current (CC) mode. In the CC mode, the output current Io is equal to Imax. For example, if the output terminal of the system is connected to a discharged battery, the system operates in the CC mode.
FIG. 4 is a simplified diagram showing a conventional flyback power conversion system with primary-side sensing and regulation. The power conversion system 300 includes a primary winding 310, a secondary winding 312, an auxiliary winding 314, a power switch 320, a current sensing resistor 330, an equivalent resistor 340 for an output cable, resistors 350 and 352, a rectifying diode 360, and a controller 370. For example, the power switch 320 is a bipolar transistor. In another example, the power switch 320 is a MOS transistor.
As shown in FIG. 4, the auxiliary winding 314 is magnetically coupled to the secondary winding 312, which, with one or more other components, generates the output voltage. Information related to the output voltage is processed by a voltage divider of the resistors 350 and 352, and is used to generate a feedback voltage 354, which is received by a terminal 372 (e.g., the terminal FB) of the controller 370. The controller 370 samples and holds the feedback voltage 354, and the sampled voltage is compared with a predetermined reference voltage (e.g., V_REF). The error of the sampled voltage with respect to the reference voltage is amplified, and the amplified error is used to control the pulse width for pulse-width modulation (PWM) and/or the switching frequency for pulse-frequency modulation (PFM) in order to regulate the output voltage in the constant voltage mode. In contrast, in the constant current mode, the output current is estimated by sensing the primary current that flows through the primary winding 310 and determining length of the demagnetization period.
FIGS. 5(A), (B), and (C) are simplified diagrams showing certain conventional timing diagrams for a flyback power conversion system with primary-side sensing and regulation that operates in the discontinuous conduction mode (DCM), the continuous conduction mode (CCM), and the quasi-resonant (QR) mode, respectively.
As shown in FIG. 5(A), in DCM, the off-time of the switch, Toff, is much longer than the demagnetization period, Tdemag. The demagnetization process ends at point C, and the next switching cycle starts after the completion of the demagnetization process. The demagnetization period is determined as follows:
                              T          demag                =                                            I              sec_p                                      (                                                V                  o                                /                                  L                  s                                            )                                =                                                    I                sec_p                            ×                              L                s                                                    V              o                                                          (                  Equation          ⁢                                          ⁢          5                )            
where Vo is the output voltage, Isec—p is the peak value of the secondary current that flows through the secondary winding, and Ls is the inductance of the secondary winding.
Additionally, as shown in FIG. 5(B), in CCM, the next switching cycle starts before the demagnetization process is completed. In CCM, the residual energy reflects back to the primary winding and appears as the initial primary current, Ipri—0, at the beginning of the next switching cycle.
Moreover, as shown in FIG. 5(C), in the QR mode, the demagnetization period, Tdemag, is slightly shorter than the off-time of the switch, Toff. The demagnetization process ends at point C, and the next switching cycle starts shortly after the completion of the demagnetization process. The next switching cycle starts at a minimum voltage level (e.g., a valley) of the drain voltage of a MOS transistor switch or at a minimum voltage level (e.g., a valley) of the collector voltage of a bipolar transistor switch.
The conventional power conversion system with primary-side sensing and regulation often operates in the DCM mode. But the CCM mode and the QR mode usually can achieve higher efficiency than the DCM mode. Hence it is highly desirable to improve the techniques of constant current control with primary-side sensing and regulation that can operate in the CCM mode and the QR modes, in addition to the DCM mode, and that can provide both high power factor and precision control of constant output current.